I am writing this post in order to get a better understanding on what in linguistics is often called "Permission causative" and "Coercive Causative" with intransitive verbs. I'll try to write as clearer as I can.

The fact is that I have no problem when dealing with sentences like the following ones >

When you make them causative, you can use を.
母は 私を 学校に 行かせる
母は 私を 友達と 会わせる

However, in the colloquial usage, に is also acceptable in many cases.
母は 私に 学校に 行かせる
(It sounds slightly odd because of the double に, though)
母は 私に 友達と 会わせる
(It sounds off depending on the context, however, it’s used in many cases)

A: 母は妹を買い物に行かせます
B: 母は妹に買い物に行かせます
I’d say A might sound more proper although B is usually acceptable in our daily life. I'm not so sure but I suppose it's grammatically correct as well. (I don’t know what “the agent” means here) Nuance-wise...well...I have to say it depends on the context or the position of the accent if it's spoken....Sorry. I appreciate if someone could help me.

Nonetheless, there are a few exceptional transitive verbs such as 待つ or 期待する or 失望する that take を instead of に for some reason. It seems tough to explain why. Here is a link to the relatively straightforward explanation in Japanese, though. http://nihon5ch.net/contents/ch5/kosatsu/43.html
Give me some time if you may.

Hi, Minarai.
To begin with, I want to let you know that I was glad to know that you were studying Japanese and that I noticed your Japanese has improved very much!

Regarding the を/に problem, I have realized that it is a very difficult issue even for us native speakers.
I’ve found out an interesting resource about this subject. Unfortunately it is written in Japanese, however, I think it is informative for many advanced learners.http://opac.kansaigaidai.ac.jp/cgi-bin/ ... earch='日本語 文法 使役'
This essay analyzes the usage of causative form of Japanese intransitive verbs mainly for the teachers who teach Japanese to foreign students.
Let me roughly introduce the contents.

1. Introduction
Even though the causative form is a subject of JLPT-3 level, many teachers would have found that the explanation of the common textbooks is just tricky or confusing.

2&3. Common definition of the particles in the causative form
The author shows how it is explained in some major textbooks. For instance, in “日本語基本文法辞典” (1986), it explains, quote “When the main verb is an intransitive verb, the cause is marked by either o or ni …When ni is used the cause has taken an action intentionally. O, on the other hand, can be used regardless of the causee’s volition, …”
In “みんなの日本語” (1998), it explains, quote, “自動詞なら the subject of an action （被使役者）には「を」が付き、他動詞のときは「に」が付く”
In “Yookoso!” (1994), it explains, quote, “自動詞の場合「を」「に」どちらも可能だが、「を」のときは coercive causative で無理矢理というニュアンスがある。一方「に」のときはしたがる被使役者にすることを許可したという意味がある。他動詞の場合は「を」しか取れないので ambiguious”.

No2 is the exceptional transitive word. The point is that, meaning-wise, it has already stated that Bob makes Annie wait in this declarative form.
＝「Bob は Annie を 待たせる」

Yes, it looks exceptional because it uses を. But did you notice that there is no Cathy here?
So, if you want to make a “causative form” in the same pattern as No1, it has to be:
「Cathy は Annie に Bob を 待たせる」
Now it doesn’t look exceptional, right? It takes に as usual.

Now, let’s take a look at slightly different pattern.

3. 「Annie eats Bob」
4. 「Annie waits for Bob」

The causative form is to be:
3 → Bob makes Annie eat Bob (himself).
「Bob は Annie に Bob を 食べさせる」
→ 「Bob は Annieに 彼を食べさせる」
4 → Bob makes Annie wait for Bob.
「ボブ は アニーに ボブを 待たせる」
→ And, we usually omit the first アニー[にボブ]を待たせる part because it sounds just redundant.
→ 「ボブは アニーを 待たせる」
↑
It looks like exceptional because it takes を. But grammatically speaking, the idea is very clear. Besides, this usage can imply that Bob makes her wait for "nobody but himself". You see? If you say "Bob は アニーに 待たせる", the listener would just wonder "wait for whom?"

Hi Nilecat, thanks so much for replying. I didn't manage to write sooner because of my studies, sorry. I will read your posts as soon as I can and I will anwer you back later. I need some time to think about this topic so as to fully understand it.

Nevertheless, today I've taken the Japanese test I was talking about... hoping that everything will be ok.

Ranja wrote:I know that the verb 待つ does have a causative form at times.

But in your first post, while I was explaining about the causative form, you started your post with this:

Ranja wrote:待たす or 待たせる is not the causative form of 待つ

What is the causative form then? That’s what I pointed out.
Now you say:

Ranja wrote:Perhaps I should have written 'normally used 待たす or 待たせる is not the causative form of 待つ'.

Not "perhaps". And your difinition of "normal" is too ambiguous. You HAVE TO write your statement clearly if you want to get in on a serious discussion because we were discussing about the usage here. Otherwise, I assume you as a rude intruder or just a troll. That’s what I think when I read your affirmative posts.

Ranja wrote:But I'm sure what I wrote was not "incorrect".

Unfortunately, that was incorrect. If you don't understand the meaning of being incorrect, please look it up in a dictionary.

Don't take this personally, but I don’t know if your mother language is Japanese as you claim.But If you think you can not properly evaluate the legitimateness of some information on your own, I recommend you not to post your personal opinions about proper grammar issue rather than exclaiming the accuracy of Wikipedia.